The present invention relates to a controller for use with an automatic transmission used in a power transmission mechanism in which driving force of a vehicle engine is converted for transmission to wheel shafts by means of the automatic transmission and more particularly, to an automatic transmission controller suitable for use with an automatic transmission provided with a direct coupling mechanism which mechanically transmits driving force of the engine to a transmission.
Some types of automatic transmission for vehicles using a torque converter adapted to transmit or couple driving force through the medium of fluid are provided with a lockup mechanism which transmits the driving force by mechanically coupling the engine output to the transmission without resort to intervention of fluid in order to improve fuel economy. In the lockup mechanism, the engine is directly coupled to the transmission by press force of clutch controlled by a working pressure. A halfway clutching state can be allowed by adjusting the working pressure on the clutch. Typically, with the aim of preventing a large shock from being generated owing to abrupt engagement of the clutch during a shift from fluid coupling state to direct coupling state, the working pressure is gradually increased to suppress the shock. Similarly, during a shift from direct coupling state to fluid coupling state, the working pressure is gradually decreased to relieve a shock.
In a conventional automatic transmission provided with the lockup mechanism, the initial value of working pressure during the shift from fluid coupling state to direct coupling state or vice versa is always kept to be constant regardless of various running conditions. Further, in the course of the shift between the direct coupling state and the fluid coupling state, the increasing and decreasing rates of the working pressure are constant.
JP-A-11-37279 discloses control of an automatic transmission having a continuously variable transmission and a lockup mechanism in combination. In the control described in this literature, the lockup state is released forcibly when the load decreases abruptly during the lockup. JP-A-4-175571 discloses that coupling force of a lockup clutch is so controlled as to be increased gradually. JP-A-3-260466 and JP-A-7-198034 disclose that coupling force is feedback-controlled such that a lockup clutch is brought into a slip state.